The present invention relates to mixers and, more particularly, in certain embodiments, to mixers for blending particulates, or fluid into a fluid stream.
Traditional oil field fracturing blenders are open top mixing systems that require sophisticated fluid control systems to maintain a nominal level of fluid in a mixing tub. The typical open tub fracturing blender in oil field services utilizes an atmospheric pressure open top blending vessel to blend particulates with carrier fluid (usually a viscous polymer fluid system). The level of the fluid in the blending vessel is controlled by various control valves and level sensors through proprietary computer software control systems. Although advancements have been made in providing a rugged, tough, responsive fluid level system, the system is still a major cause of critical equipment failures on the fracturing blenders. In order to eliminate these components and systems, centrifugal type, closed system blenders have been used.
The typical centrifugal blending system utilizes a minimal volume mixer case to collect particulates and carrier fluid and redirect them to the mixer discharge. These systems typically use a combination centrifugal force impeller to inject the particulates and provide carrier fluid under pressure to the mixer. In addition to creating pressure, the centrifugal force on the carrier fluid in the mixer prevents the carrier fluid from exiting the mixer. The particulates enter the mixer at an eye of a rotating impeller, which provides motive force to move the particulates into the mixer and prevent the pressurized carrier fluid from escaping to the atmosphere. The carrier fluid section or the mixer impeller must provide sufficient flow at the pressure required by high-pressure downhole pumps (typically 50 to 75 psi). The particulates section of the pump impeller must be able to inject particulates into the pressurized mixer and keep the carrier fluid contained. In some cases, an external boost pump (such as a low pressure, high volume axial flow pump) is used to provide efficient suction characteristics to keep the carrier fluid section of the mixer impeller primed. However, these high mix pressures, which require a high mixer rpm, may cause severe erosion on mixer rotating components due to the high velocities of abrasive fluids.
Generally, the centrifugal mixer volume is kept small to minimize required wall thickness (required by the typical operating pressure range of 50-70 psi.), along with associated weight and cost. For example, for 50-70 psi operating pressure, the volume of the mixer is typically less than two barrels. This small volume prevents significant dwell times. For example, at 50 barrels per minute, the dwell time of a 2 barrel volume is less than 2.5 seconds. Thus, when abrupt changes occur in the carrier fluid (e.g. slurry or water) supply or particulate delivery rate, (i.e., sand-off, empty frac tank, etc), the concentration of particulates in the mixer can become extremely high or low before the control system can properly respond to the abrupt change. Thus, fluctuations in the carrier fluid delivery system (e.g., the slurry delivery system and/or the water supply system), or the particulate delivery system can be catastrophic, even causing the entire fracturing job to fail, requiring extensive rework.
Further, when throughput is slowed, and the fluid velocity drops below the minimum particle carrying velocity, there is a tendency for the particulates to “fall out” of the carrier fluid. When downhole rate stops, the mixer may deadhead under mixing pressure, and any slurry in the mixer will tend to separate. This necessitates a flush of the mixer before mixing is stopped, so that there is a clean fluid plug when mixing resumes. Additionally, getting particulates into the mixer vanes may be very difficult. Particulates are directed from vertical to horizontal and accelerated to enter the vanes. Thus, the vanes are either very deep or inducer vanes are used. Finally, this design lacks an atmospheric pressure tub to provide for removal of entrained air in the downhole pressure piping, necessitating a connection to an external holding tank to allow the high pressure pumping units to “prime-up” or recirculate fluid to remove entrapped air.